1. Field of the Invention
This invention relates generally to a magnetic powder for high density magnetic recording and more particularly to production of a magnetic powder having a high dispersibility suitable for vertical magnetic recording.
2. Description of the Prior Art
Magnetic recording has been based on magnetization along the longitudinal direction of the plane of the recording medium. However, when an attempt is made to perform high density recording in such a system, the demagnetizing field within the recording medium increases. Thus, it is difficult to achieve very high density recording with this recording system.
In order to eliminate such a difficultly, a vertical magnetic recording system has recently been proposed which is based on magnetization along the vertical direction of the plane of the recording medium. In such a recording system, the demagnetizing field within the recording medium decreases even when the recording density is increased, so that this system is quite suitable for high density recording. With the vertical magnetic recording system, it is necessary that the axis of easy magnetization be normal to the surface of the recording medium. Among such recording media, there is known a recording medium which is obtained by mixing magnetic particles with a binder, coating the mixture on a non-magnetic tape, and introducing the tape into a magnetic field such that the plane of the tape is normal to the direction of the magnetic field. As a result, the axis of easy magnetization of the magnetic particle aligns along the direction of the magnetic field. After drying, a recording medium suitable for vertical recording is obtained.
As the magnetic particles, hexagonal ferrites such as barium ferrite (BaFe.sub.12 O.sub.19) are usually used. These hexagonal ferrite particles are in plate form and the axis of easy magnetization is normal to the plane of the surface so they are advantageous in that vertical orientation may be easily accomplished by magnetic field orientation processing or mechanical processing. However, in order to use the hexagonal ferrites as a vertical magnetic recording powder, certain conditions must be satisfied.
For example, the hexagonal ferrites are too high in coercive force iHc (generally over 5,000 oersteds) to be recorded by the usual magnetic head. Thus, it is necessary to reduce the coercive force to a value suitable for vertical magnetic recording.
Further, it is preferred that the crystal size of the hexagonal ferrites be controlled within a range of 0.01-0.3 .mu.m for vertical magnetic recording. When the crystal size is less than 0.01 .mu.m, the ferromagnetism necessary for magnetic recording is not obtained, and when it exceeds 0.3 .mu.m, high density magnetic recording cannot be achieved.
It is further required that the hexagonal ferrites be homogeneously dispersed in a medium such as a binder or paint. Therefore, it is necessary that individual ferrite particles not aggregate at least during preparation thereof.
A hexagonal ferrite having a relatively low coercive force and the desired particle size can be obtained by a glass crystallization technique, using hexagonal ferrite-forming components including a coercive force-reducing element as a starting material together with a glass-forming component. However, it has often been found by the present inventors that even a hexagonal ferrite obtained by such a glass crystallization technique could not be uniformly dispersed in a binder or paint, thus resulting in poor vertical alignment and a low squareness ratio of magnetization curve.